
The present invention relates to a method and apparatus for determining the location in space of an end point of a multi-section, articulated arm assembly.
Many procedures require an accurate location of a point in a coordinate system. For example critical medical procedures such as endoscopy, image guided surgery, or catheterization require a device and method for precisely locating a tip of a surgical instrument in three-dimensional space, either on the outer surface of a patients body or within the patient""s body. Various techniques have been developed to perform this function in the prior art, based on the detection of changes in the strength and phase of electromagnetic fields, using a collection of optical sources and detectors arranged in a line of sight system, or using a mechanical arm with multiple straight sections and rotary encoders located at the mechanical joints between adjacent pairs of arms.
The electro-magnetic approach, examples of which are U.S. Pat. Nos. 5,568,809 and 5,600,300, measures a change in an electro-magnetic field due to changes in relative position and orientation between a transmitter and a receiver. This approach suffers from inaccuracies due to the inevitable presence of metals in the vicinity. For example, in an operating room environment, surgical instruments, illumination fixtures, tables, headrests, and immobilization and support structures all potentially interfere with the electromagnetic approach. As a result, it is difficult for an electro-magnetic based positioning system to work reliably and accurately in a wide variety of settings in the presence of metals.
The line of sight approach determines the position and orientation of the tip of the assembly by pulsing the optical sources in a predetermined fashion and measuring the arrival times of the pulses at a multiplicity of optical detectors attached to the tip. This approach may be awkward to use in certain situations since it requires a user of the system and any attending personnel to take adequate care not to obstruct the detectors from the multiple optical sources. Proper location of the sources will avoid this in most instances.
In the mechanical approach, a series of coordinate angles between pairs of straight sections are measured by placing rotary encoders at the appropriate joints. The angle data is then used to calculate the tip position. The sheer size and limited range of motion typical of mechanical arm devices makes this approach cumbersome to use in a some environments.
It is therefore an object of the present invention to provide a probe measuring system in which the above disadvantages are obviated or mitigated.
In one aspect of the invention there is provided a probe position sensing system for determining a spatial location in a coordinate system of a portion of a probe assembly. The probe assembly includes an articulated arm having a pair of sections interconnected by a flexible joint and at least one element extending through the joint and positioned to be subjected to a degree of flexure due to relative displacement of the sections. A flexure of the element induces a change in a physical property associated with the element. The system includes an instrument to monitor the physical property and to derive an angle between adjacent sections from variations of the physical property.
In a further aspect of the invention, the probe position sensing system includes a stationary unit. The probe assembly connected to the unit and the probe assembly includes at least one articulated arm having a pair of sections interconnected by a flexible joint. At least one element extends through the joint and is positioned to be subjected to a degree of flexure due to relative displacement of the sections. A flexure of the element induces a change in a physical property associated with the element. At least one instrument is included in the system to monitor the physical property and to derive an angle between adjacent sections from variations of the physical property.
An additional aspect of the invention provides a method of determining a spatial location in a coordinate system of a portion of a probe assembly in a probe position measuring system. The system has a plurality of sections interconnected by corresponding flexible joints of known geometrical parameters. The method includes the steps of; identifying a predetermined fiducial location in the coordinate system; positioning the portion of the probe assembly at a desired location; measuring a variation of at least one property in a plurality of elements extending through the joint to determine the angular displacement of the arm from the predetermined fiduciary position; transmitting of the angle information to a computer; determining the spatial location of the portion of the probe assembly by combining the angle information and the geometrical parameters.
In a further aspect of the invention there is provided an articulated arm including a pair of sections and a flexible joint extending between and connecting the pair of sections. An element is located in the flexible joint and positioned to be subjected to a degree of flexure due to relative displacement of the sections. The element has a first longitudinal neutral axis and a sensor element having a second longitudinal axis contained in the element. The second longitudinal axis is in a spaced apart radial relationship with respect to the first longitudinal axis, wherein flexure of the sensor element induces a variation in a physical property associated with the sensor element, the variation used to derive an angle between adjacent sections.
The method may also include the steps of displaying the spatial location of the portion of the probe assembly on a display, superimposing an image on the display with the spatial location of the probe assembly, and following the movement of the image and computing the relative difference between the spatial location and the image.